The largest classroom teaching union is considering balloting its members on a boycott of national tests for seven-, 11- and 14-year olds, which could lead to hundreds of thousands of youngsters not sitting them next spring.

Doug McAvoy, the general secretary of the National Union of Teachers, said his union was reviving its campaign for both the tests and league tables to be scrapped on "educational" grounds.

A survey of more than 3,000 primary and secondary school teachers in England showed a vast majority continued to oppose key stage 1, 2 and 3 Sats (standard assessment tasks) tests, a decade after they were introduced by the Conservative government.

Mr McAvoy said the NUT would try to enlist the support of parents, school governors and the other classroom unions in England and Wales in favour of a boycott, which might lead to a ballot before the next tests in May 2003. He said: "There is a compelling educational argument for getting rid of Sats and league tables. Our task now is to take that argument to parents and governors and to seek to persuade the other teacher organisations that we should be preparing for a boycott of these tests." That might lead to a ballot of members some time next term, he explained.

The last time teachers took such action was in 1993 when the NUT and the National Association of Schoolmasters Union of Women Teachers refused to have their pupils take the tests.

Mr McAvoy admitted yesterday that as the tests were now more established, it might be "more challenging" to get the support of parents.

The NUT's survey was carried out during the summer term, and the results analysed by Sean Neill of Warwick University's institute of education. It revealed that more than 84% of members supported a boycott of the tests because of the "narrowing effect" they had on the curriculum. But 52% said they would take part in a boycott only if other teachers' organisations were also involved.

Testing the youngest pupils at the age of seven triggered the strongest opposition, with 92.1% saying they would support a refusal to put them through key stage 1 exams.

More than eight out of 10 said internal assessment by teachers was more useful to pupils, parents and schools. Tests at 11 are critical as they are used to rank primary schools in national league tables, but 87% of teachers said they favoured a boycott.

A significant proportion of teachers was concerned about the accuracy of the external marking of the tests - 40.3% said that external marking was not accurate or reliable, with a further 43% saying they had mixed views.

Mr McAvoy said it was "illogical" that England still had the tests. In Wales, key stage 1 tests had been scrapped, although children were still tested at 11 and 14. Scotland and Northern Ireland have never had compulsory national tests.

The results of separate ballots of NUT and NASUWT members over the London allowance are due on October 29.

A spokesman for the Department for Education and Skills said: "If we want a world-class education system then we need to test how children are performing.

"We make no apologies for publishing the results of tests. We are not going back to the days of the 1970s when parents had little or no information about the education of their children.

"It is only right that they and others can see how well schools and the education system are performing."